Naive CD4+ T cells produce interleukin-2 (IL-2) upon stimulation by antigen and antigen-presenting cells but can be induced to differentiate into cells that principally produce interleukin-4 (IL-4) or interferon gamma (IFNgamma). The determination of which lymphokine is dominantly produced has a major impact on the protective value of the immune response. Utilizing CD4+ T cells from T cell receptor transgenic mice, it was shown that IL-4 itself is the principal factor determining whether T cells develop into IL-4 producers and, in addition, strikingly suppresses priming for IFNgamma production. In the absence of IL-4, priming leads to cells that produce moderate amounts of IFNgamma. Addition of IL-12 to the priming culture upregulates priming for IFNgamma production and allows such priming to occur even in the presence of IL-4. The molecular basis of such priming is unknown but it appears likely that it involves regulation of lymphokine transcription. A study of the IL-4 promoter revealed that, in transient transfection assays using a CAT reporter gene, the sequence from -87 to the transcription start site exerts fully inducible IL-4 promoter-mediated CAT transcription in EL-4 thymoma cells that have been stimulated with phorbol esters. Two regions that contain copies of a consensus sequence (CS1 and CS2 - ATTTTCCNNTG) have been identified. Deleting CS2 causes a modest diminution in CAT transcription while deleting CS1 completely inhibits transcriptional activity. PMA- inducible binding factors that interact with CS2- and CS1-containing oligonucleotides have been identified; these appear to be different from one another. Multimerization of the CS1-containing oligonucleotide drives CAT transcription from a minimal promoter in TH2 but not TH1 cells suggest-ing this region may be critical to these distinctive lymphokine- producing phenotypes of the two cells. IL-4 action is mediated by binding to a high affinity receptor. This binding results in the phosphory-lation of a 170 kDa substrate, designated 4PS, an analog of the insulin receptor substrate-1 (IRS-1). Phosphorylation of 4PS/IRS-1 is lost in truncation mutants of the receptor that delete the most mem-brane proximal tyrosine. The sequence around that tyrosine is homologous to the sequence around a comparable tyrosine in the insulin and IGF-1 receptors; this motif has been termed the I4R motif. A fusion protein containing the I4R motif binds the substrate and the kinase that can catalyze the phosphorylation of IRS-1.